Pulmozyme (dornase alfa) is a highly purified solution of recombinant human deoxyribonuclease 1 (rhDNase), an enzyme with the specific function of cleaving the extracellular DNA which contributes to the viscosity of airway secretions in cystic fibrosis (CF). Pulmozyme is a liquid delivered by aerosol and has been shown to be safe and effective in the treatment of CF lung disease. The relationship between particle size and drug deposition is still under debate. However, it is clear that smaller particles penetrate deeper into the respiratory tract. This study compared two compressor/nebulizer systems with different particle of mass mean diameters to determine the optimal system for delivery of Pulmozyme. The efficacy objectives were to compare short-term improvement (14 days) in FEV1, FCV, and FEF25-75, produced by Pulmozyme administered via one of the two nebulizer systems. The safety objective was to compare the number and percent of adverse events in subjects receiving Pulmozyme via one of the two nebulizer systems. Pulmonary function testing was performed once before treatment (day 1) and once at the conclusion of treatment (day 14). Serum was drawn once before and once after the 14 day treatment period to measure serum DNase levels. Subjects' logs and interval histories documented adverse events. This multicentered study was conducted in the United States and Canada and involved 800 stable CF patients with FVC>70% of predicted, who had not previously received Pulmozyme. At Children's Hospital of Philadelphia, 20 patients were enrolled and completed the study over a four month period. All the data has been submitted to the sponsor for analysis, and it is expected that results will be available within the next two months.